Implement the core of let/const statement. (#3239)

This patch implements the core part of let/const statements. Redeclarations are correctly
detected and separate contexts are correctly created for these statements. Register
optimizations are also emplyed whenever possible.

Lots of features are still missing:
 - checking the var statements in eval
 - const are treated as lets
 - single statement checks are missing
 - export declarations are exported as vars, let/const export is not supported

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

jerry-core/parser/js/js-parser.h

@@ -112,6 +112,9 @@ typedef enum
   PARSER_ERR_INVALID_RIGHT_SQUARE,                    /**< right square must terminate a block */
   PARSER_ERR_DUPLICATED_LABEL,                        /**< duplicated label */
   PARSER_ERR_OBJECT_PROPERTY_REDEFINED,               /**< property of object literal redefined */
+#if ENABLED (JERRY_ES2015)
+  PARSER_ERR_VARIABLE_REDECLARED,                     /**< a variable redeclared */
+#endif /* ENABLED (JERRY_ES2015) */
 
 #if ENABLED (JERRY_ES2015)
   PARSER_ERR_MULTIPLE_CLASS_CONSTRUCTORS,             /**< multiple class constructor */